For example, as a detector of alpha rays, there is known a detector using a ZnS scintillator that emits light in response to incident alpha rays. Further, there is known a radiation detector that utilizes a fact that alpha rays make nitrogen in the atmosphere emit light to observe light emission of nitrogen and to thereby detect alpha rays.
There is known, as the radiation detector that observes light emission of nitrogen, a radiation detector including a collecting lens that collects light based on the light emission of nitrogen, a wavelength selecting element that extracts the light emission of nitrogen from the collected emission light, an optical element that separates the extracted light emission of nitrogen into transmitted light and reflected light, an optical element that changes a propagation direction of the reflected light, photo detectors that receive respectively the transmitted and reflected lights and count the number of photons, and a signal processor that detects the light emission of nitrogen caused by alpha rays based on simultaneous measurement of the transmitted light and the reflected light by the photo detectors (refer to, for example, Non-Patent Document 1: Remote Optical Detection of Alpha Radiation, IAEA-CN-184/23, the entire content of which is incorporated herein by reference.
According to the conventional technology disclosed in Non-patent Document 1, the light emission of nitrogen is selectively extracted by the wavelength selecting element and then branched into two lights. For the two lights, the number of photons is counted using the two photo detectors. At this time, the two photo detectors each also detect a noise signal due to ambient temperature or radiation. The light-emitting photons of nitrogen are observed simultaneously in the two photo detectors, while the noise signals detected in the respective two detectors are temporally independent of each other. Thus, the signal processor can detect a signal corresponding to the light emission of nitrogen by extracting a signal measured simultaneously by the two detectors. As a result, it is possible to selectively observe the light emission of nitrogen to thereby allow detection of alpha rays.
However, in the above-described radiation detector, the photon counting is performed for the branched two emission lights of nitrogen caused by alpha rays using the two photo detectors, so that when the number of alpha rays emitted is small and, accordingly, the number of light-emitting photons of nitrogen is small, detection of the photons becomes difficult.